FIG. 4 is a diagram showing a circuit configuration of an example of a related electric power converting system of this kind (see JP-A-2007-181331, for example).
In the diagram, reference numeral 1 denotes an AC power supply such as a commercial power supply and reference numerals 2 to 5 denote AC motors as loads, to which electric power is supplied respectively from an electric power converting system 10.
The electric power converting system 10 shown in FIG. 4 is formed of an operation sequence circuit 11 conducting the whole operation sequences of the electric power converting system 10, a master disconnecting switch 12, an electromagnetic contactor 13, a converter 14 with diodes in bridge connection, a DC reactor 15 for smoothing the output voltage of the converter 14 and inverter units 20, 30, 40 and 50 as a plurality of sets of inverters.
The inverter units 20, 30, 40 and 50 are formed of capacitors 23, 33, 43 and 53, charging current suppressing resistors 21, 31, 41 and 51, electromagnetic contactors 22, 32, 42 and 52 and inverter circuits 24, 34, 44 and 54, respectively.
Each of the capacitors 23, 33, 43 and 53 is made of a capacitor such as an electrolytic capacitor for smoothing the output voltage of the converter 14; the charging current suppressing resistors 21, 31, 41 and 51 suppress inrush currents to the capacitors 23, 33, 43 and 53.; respectively; the electromagnetic contactors 22, 32, 42 and 52 are connected in parallel to the charging current suppressing resistor 21, 32, 41 and 51, respectively; and each of the inverter circuits 24, 34, 44 and 54 outputs an AC voltage with a desired amplitude and frequency with inverse-parallel connection circuits, in which an IGBT (Insulated Gate Bipolar Transistor) and a diode are connected in inverse-parallel, arranged in bridge connection.
The AC motors 2 to 5 to which electric power is supplied by the electric power converting system 10 shown in FIG. 4 are made available for power sources such as driving sources for conveying line equipment in iron-manufacturing machinery or paper-manufacturing machinery.
In an electric power converting system such as the electric power converting system 10 shown in FIG. 4 formed with a plurality of sets of inverters whose electric power is supplied from a common DC electric power supply and which can be operated mutually independently, as is disclosed in JP-A-2007-181331, for example, various kinds of measures are taken against the case when any one of a plurality of the sets of inverters causes failure so that no malfunction due to the failure affects other inverters.
Moreover, when anyone of a plurality of the sets of inverters causes a failure to be brought into a state in which the operation of the failed inverter is stooped, it is required that the failed inverter can be immediately replaced by an inverter that can be normally operated.
In spite of such a requirement, however, in the related electric power converting system 10 shown in FIG. 4, when any one of the inverter units 20, 30, 40 and 50 causes a failure to be brought into a state in which the operation of the failed inverter unit is stopped, the failed inverter unit is replaced by an inverter that can be normally operated with replacement operations carried out as follows with the operation of the whole electric power converting system 10 being stopped. First, by the instruction from the operation sequence circuit 11, the electromagnetic contactor 13 is opened and, along with this, the operation of the whole electric power converting system 10 is stopped. Thereafter, with the master disconnecting switch 12 being opened, the failed inverter unit is disconnected before a normal inverter unit is connected.
Thus, the related electric power converting system 10 has a problem in that the operation of the whole electric power converting system must be stopped. In particular, in the case in which the. subject machinery is a line control system in iron-manufacturing or in paper-manufacturing, this causes tremendous losses. Furthermore, in such an inverter replacement operation, there was also a problem in that the replacement operation requires a certain time because it is necessary to wait for a time to elapse until charges remaining in the respective capacitors forming inverter units are sufficiently discharged.
Accordingly, it is an object of the present invention to provide an electric power converting system in which the above explained problems are solved.
Further objects and advantages of the invention will be apparent from the following description of the invention.